The contents of various nutrient elements in plant tissues vary greatly. But the importance of essential nutrients in plants is the same regardless of the content, which is the law of equal importance of nutrients. The law of equal importance shows that although the content of each element is different, it is indispensable. For example, compared with nitrogen, phosphorus, potassium and other large elements, the content of trace element molybdenum in plants is very small. However, plants lack molybdenum, their growth is hindered, and symptoms (such as citrus macular disease and cauliflower whiptail disease) appear; What's more, the growing point is necrotic and the whole plant dies, and its adverse consequences are exactly the same as the lack of a large number of elements. This law is of great significance to guide production. If we only emphasize the supply of one element and ignore the effects of other elements on plants, it will inevitably lead to nutritional imbalance, thus affecting the normal growth of plants.

The irreplaceable law of nutrient elements means that every essential nutrient element in plants has its special function and cannot be replaced by other elements. A plant is necessary for a certain nutrient element, because it has one or more special functions and is irreplaceable by other elements. However, this does not mean that there are no similarities between elements, or even no similarities. In fact, some elements have similar functions in plant metabolism, that is, they can all play similar roles in a certain metabolic process or a certain part of a metabolic process, so they can partially replace each other. A prominent example is the similarity between potassium and sodium in some nutritional functions. An important function of potassium is to regulate osmotic pressure in plants, and sodium also has this function. When potassium is insufficient, sodium can replace potassium to adjust osmotic pressure and let potassium perform other more important physiological functions. The principle of partial substitution of potassium and sodium has certain significance in growth. Some sodium-loving crops (such as sugar beet and barley) can be partially replaced by sodium, which not only saves the amount of potassium fertilizer but also does not cause crop yield reduction. However, it should be pointed out that the similarity or substitution between these elements is only partial and secondary, and most nutrient elements have their special functions in plants and cannot be completely replaced by other elements.